Springs are installed in upholstered chairs to give you a firm or springy seat. Most springs are coated to prevent corrosion and made of tempered steel of varying gauges. For a softer seat, you'd have a higher gauge (11 gauge, for example) and for a firmer seat, a lower gauge of steel wire would be used for the spring. Three main types of springs are used in chair seats: coil, sinuous and marshal.
Coil springs (also called eight-way knotted or tied springs) are found in antique and new high-end chairs. The height and gauge of the spring will depend on the height of the chair seat. Typically, regular or medium tension springs are used and often outlive the life of the chair they're installed in. Coil springs are tied together using a four-way knot system and twine. Eight knots are secured to each spring, usually with two going front to back, two going side to side and one knot between each of those four, making eight knots total. Coil springs prevent sagging, no matter how heavy the person who sits on the chair.
Sinuous springs (also called zigzag springs) aren't as durable as coil springs, but they are more commonly used in modern chairs and other upholstered furniture because they cost less to make. Sinuous springs are produced in rolls and cut to the length needed. They are called ziz-zag springs because the band of metal is bent like an "S," or in a curving zigzag type of line. Sinuous springs are attached to the front and back rails of the chair and, when linked together, are able to support the weight of someone sitting on them. Sinuous springs are commonly found in chairs, ottomans and sofas and are installed in both the seats and the backs. The firmness of a seat that uses sinuous springs depends on the gauge of the wire used. Nine gauge, for example, is used for firm seats and ottomans, while 11 gauge would provide a softer seat and is often used for chair and sofa backs. This type of spring system is ideal for furniture that is lower to the ground and is found often in contemporary chair designs.
Marshall springs are typically used in mattresses but are also sometimes used in cushions. The spring is enclosed in fabric, and the steel is tempered and coated to prevent corrosion. Marshall springs are made of coils of springs which are placed in separate pockets and then joined to form cushions. When precompressed, marshal springs give a firmer seat or mattress.